This invention pertains generally to electrical resistors and particularly to resistors of such type formed on a substrate.
It is known in the art that great difficulty may be encountered when it is desired to match the temperature coefficients of resistance of so-called thick film resistors. That is to say, when two (or more) thick film resistors having greatly differing values of resistance are formed on a substrate, it is very difficult to obtain resistors with similar temperature coefficients of resistance. Thus, if it is desired to fabricate a resistive voltage divider with a very great difference between the resistances of two resistors making up such a divider, known design techniques cannot be followed to produce an accurate resistive voltage divider for use in applications wherein the ambient temperature may change within wide limits.
Many of the factors which contribute to the difficulty of designing and making satisfactory thick film resistors are known. Thus, it is common practice to use resistive materials having inherently low temperature coefficients of resistance processed under identical conditions and dimensioned so that the physical dimensions of the resistors are as nearly identical as possible. As noted above, however, when the resistances of two resistors must differ greatly, if known design methods are followed, the physical dimensions of the two must differ significantly in some respect with the result that tracking of their thermal coefficients of resistance within close limits may not be achieved.
Another difficulty experienced with any known thick film resistive voltage divider is that, during processing (furnace firing), the mechanism of diffusion may take place at the junctions between the film material and the terminals of the divider. Such a process then causes the value and thermal coefficient of resistance of each thick film resistor to change in accordance with the length of each such resistor. The result, therefore, is that the divider ratio of any known thick film resistive voltage divider may change whenever the ambient temperature changes.